The present invention is drawn to an electrochemical sensor. More specifically, a separation device having a non-axial flow path therethrough for use within a reference cell is disclosed.
An electrochemical sensor used for measuring pH, ORP, or other specific ion concentrations is typically comprised of three parts: a specimen sensing ion electrode, a reference cell, and an amplifier that translates signal into useable information that can be read. For example, in the case of a pH sensor, the specimen sensing ion electrode can be a hydrogen ion sensitive glass bulb with a millivolt output that varies with the changes in the relative hydrogen ion concentration inside and outside of the bulb. Conversely, the reference cell output does not vary with the activity of the hydrogen ion.
The reference cell is the structure in which most problems can occur within an electrochemical sensor. The reference cell consists of essentially three parts: an internal element such as a metal-metal salt, e.g., Ag/AgCl, Pt/Hg2Cl2, etc., a filling solution such as an electrolyte, and a liquid junction through which the filling solution contacts the desired specimen to be measured.
Specifically, the reference cell is used to maintain a common electrical potential with the specimen fluid being measured. The filling solution or electrolyte provides the conductive bridge to the specimen fluid and surrounds the reference element with an electrochemically stable environment. In order to obtain an accurate reading, this liquid junction must be in place. In the ideal liquid junction, electrolytic contact between the reference element and specimen fluid would provide the necessary communication, and yet prevent mixing of the specimen fluid with the electrolyte. However, liquid junctions can not be perfect. This is because contact between the electrolyte and the specimen fluid is present in order for ion flow to occur, and thus, mixing can ultimately occur.
With earlier pH meters, the liquid junction was simply a minute opening in a glass or ceramic barrier through which ion communication between the two solutions could be established. However, with prolonged usage, the single opening junctions were found to become readily clogged. Thus, more recently, liquid junction designs have typically comprised of ceramic or other frit material, fibrous material such as quartz, or sleeve junctions. Porous materials such as wood, Teflon(trademark), wicks, or ground glass points have also been used.
In U.S. Pat. No. 3,440,525, the use of a large junction surface comprised of wood or a porous ceramic material is disclosed. It turns out that wood in particular is a good material for use because electrolyte contact can be maintained through small capillaries or natural channels which extend axially (in the direction of the wood grain) between the electrolyte and the specimen fluid. Though the use of wood or other porous materials provides an effective liquid junction, it became desirable to extend the life of various electrochemical sensors by prolonging the usefulness of the wood or other fibrous material used in the sensor.
In U.S. Pat. No. RE. 31,333, the use of a combination of larger wooden plugs linked by smaller wooden plugs is disclosed. An adhesive sealant such as epoxy is used to seal the abutting end surfaces of the large plugs prior to assembly. Thus, when the wood plugs are assembled and filled with electrolyte and the epoxy is in place, the path for ion flow is non-linear. In other words, due to the presence of the epoxy barriers, the ions must pass back and forth between a series of non-axially arranged wood plugs.
In U.S. Pat. No. 5,630,921, an electrochemical sensor is disclosed comprised of a first longitudinal series of semipermeable plugs impregnated with an electrolyte, a second series of semipermeable plugs disposed in an overlapping relation ship with the first series with an interlocking fit, and a series of impermeable plugs. Plugs from the second series of semipermeable plugs pass through the impermeable plugs to maintain an ionic path. Thus, though impermeable plugs are used to retard the poisoning of the reference cell, the ionic path is maintained by semipermeable plugs.
In U.S. Pat. No. 6,054,031, a junction for ionic communication is described which is essentially a channel that extends between an inner surface of a housing and an outer surface of an inner body. The channel is designed with a relatively small cross-section for providing ionic continuity, but also provides a very long and tortuous channel length, thus increasing the ion transit time through the channel. By using such a design, the ion exchange between solutions separated by the channel is limited or significantly slowed. This design avoids the problems associated with plugging because the cross-section can be larger than those described in previous designs. Specifically, a helical channel is disclosed that includes these properties.
The present invention is drawn to a salt bridge for an electrochemical sensor comprising (a) at least two chambers for containing an electrolyte fluid; (b) a plug for separating the at least two chambers, said plug being essentially impermeable to the electrolyte fluid; and (c) a narrow opening through the plug providing a non-axial flow path for ionic communication between the at least two chambers when the electrolyte fluid is present.
In a further detailed aspect of an embodiment of the invention, a salt bridge for an electrochemical sensor can comprise (a) at least two chambers for containing an electrolyte fluid; (b) a plug for separating the at least two chambers, wherein the plug comprises a material essentially impermeable to the electrolyte fluid; (c) an orientation axis defined by the shortest distance between the at least two chambers; and (d) a non-axial narrow opening through the plug with respect to the orientation axis, wherein the non-axial narrow opening defines a non-axial flow path between the at least two chambers, wherein at least a section of the non-axial flow path is within the non-axial narrow opening, and wherein the non-axial flow path provides ionic communication between the at least two chambers when the electrolyte fluid is present in the at least two chambers.
Additionally, a separation device for separating multiple chambers within an electrochemical reference cell is disclosed comprising (a) a first fluid impermeable barrier having a fluid directing surface and including at least one open channel for allowing ionic communication between the multiple chambers when a continuous electrolyte fluid is present; and (b) a second fluid impermeable barrier having a fluid blocking surface mated against the fluid directing surface such that the open channel is closed to form a tunneled flow path.
Further, a separation device for separating multiple chambers within an electrochemical reference cell can also comprise (a) a first fluid impermeable barrier having a fluid directing surface and including at least one open channel for allowing ionic communication between the multiple chambers when a continuous electrolyte fluid is present; and (b) a second fluid impermeable barrier having a fluid blocking surface mated against the fluid directing surface such that the open channel is closed to form a fluid flow path, wherein the first and second fluid impermeable barriers are configured in the shape of discs, each having axially centered bores, and wherein one of the discs has a larger outer diameter and a larger bore diameter than the opposing disc.
Each of these embodiments are preferably used within an electrochemical sensor for measuring ionic properties of a fluid specimen. Such a device is preferably comprised of a reference cell having a first chamber proximal to the fluid specimen desired to be measured; a second chamber distal to the fluid specimen; an essentially impermeable plug for separating the first chamber from the second chamber; a non-axial flow path through the plug which fluidly connects the first chamber to the second chamber; a continuous electrolyte fluid within the first chamber, the second chamber, and the non-axial flow path; and a liquid junction area for contacting the continuous electrolyte fluid with the fluid specimen. Additionally, an electrolyte sensing element, e.g., metal-metal salt, can be present in the second chamber and in ionic communication with the continuous electrolyte fluid, and a specimen sensing electrode can also be in electrical communication with the electrolyte sensing element such that differences in electrical potential may be measured.
A method of defining pH compared to a reference is also disclosed comprising the steps of (a) establishing a reference cell; (b) contacting a solution specimen with a specimen sensing ion electrode; (c) establishing electron flow between the reference cell and the solution specimen from a first chamber, across a small non-axial ion flow path which penetrates an otherwise impermeable plug, to a second chamber; and (d) measuring any difference in electrical potential.
In a further detailed aspect of an embodiment of the invention, an alternative method of defining pH compared to a reference can comprise (a) establishing a reference cell; (b) contacting a solution specimen with an ion sensor; (c) establishing electron flow between the reference cell and the solution specimen from a first chamber, across a small non-axial ion flow path to a second chamber, wherein the non-axial ion flow path is non-axial with respect to an orientation axis defined by the shortest distance between first chamber and the second chamber, and wherein the non-axial flow path is through a plug comprising an essentially impermeable material which separates the first chamber from the second chamber; and (d) measuring any difference in electrical potential between the reference cell and the solution specimen.